VLA Measurements of Faraday Rotation through Coronal Mass Ejections

Abstract

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the Sun that play an important role in space weather. Faraday rotation (FR) is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma such as a CME and is proportional to the path integral through the plasma of the electron density and the line of sight component of the magnetic field.FR observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch; however, separating the contribution of the plasma density from the line of sight magnetic field is challenging.We report on simultaneous white-light and radio observations made of three CMEs in August 2012. We made sensitive Very Large Array (VLA) full-polarization observations using 1 − 2 GHz frequencies of a "constellation" of radio sources through the solar corona at heliocentric distances that ranged from 6 − 15R ⊙ . Of the nine sources observed, three were occulted by CMEs: two sources (0842+1835 and 0900+1832) were occulted by a single CME and one source (0843+1547) was occulted by two CMEs. In addition to our radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. (2007) and Jensen & Russell (2008), as well as previous CME FR observations by Bird et al. (1985).

Evidence for Rayleigh-Taylor Plasma Instability at the Front of Solar Coronal Mass Ejections

“…In order to validate the RT linear theory for the plasma perturbations observed at the CME front, an estimate of the strength of the magnetic field when perturbed by the transit of the CME, B, would be needed. There exists a growing literature on measurements of CME magnetic fields in the solar corona e.g., [47][48][49][50][51][52][53][54][55][56]. Despite the quite large variability of B estimations, due to the analysis of different events with different techniques (e.g., Faraday-rotation or moving type IV bursts observations, conservation principle of magnetic helicity, determination by means of shock properties), it can be however confidently stated that the magnetic field strength B associated to the trailing edge of a CME is less than 1 G when detected at a heliocentric distance of about 2 R .…”

Radio Observations of Coronal Mass Ejections: Space Weather Aspects

“…The linearly polarized emission from an extragalactic (or artificial source, say, a satellite beacon) rotates when crossing magnetized plasma. The effect is called Faraday Rotation (FR) and the degree of rotation, known as Rotation Measure (RM), depends on the total density and magnetic field along the path (e.g., Kooi et al, 2017 and references therein). The left and middle panels of Figure 5 show recent FR observations by the VLA of not one but two CMEs crossing over the radio source 0843 (Kooi et al, 2017).…”

Radio observatories and instrumentation used in space weather science and operations

“…As light from a distant radio source travels through the interplanetary medium (IPM), the plane of polarisation of the radio waves rotate, with the amount of rotation depending on the magnetic field strength and direction in the IPM or any CME contained within it. This can lead to a calculation of the line-of-sight magnetic field component of a CME in interplanetary space, and some promising results have recently been shown using the Very Large Array at 1-2 GHz for a CME at 6-15 R e (Kooi et al, 2017). However, observing CMEs further into the heliosphere (0.4 AU) at LOFAR frequencies must contend with the additional Faraday rotation also experienced by emission passing through the ionosphere, so distinguishing the rotation component due to a CME alone remains a significant challenge in radio space weather studies.…”